CMP-Kdo synthetases from Gram-negative bacteria activate Kdo for incorporation into lipo- and capsule-polysaccharides. Here we report the crystal structure of the capsule-specific synthetase from E. coli at 2.3 Å resolution. The enzyme is a dimer of 2 × 245 amino acid residues assuming C2 symmetry. It contains a central predominantly parallel β-sheet with surrounding helices. The chain fold is novel; it is remotely related to a double Rossmann fold. A large pocket at the carboxyl terminal ends of the central β-strands most likely accommodates the catalytic center. A putative phosphate binding site at the loop between the first β-strand and the following helix is indicated by a bound iridium hexachloride anion.
